In the integrated circuit industry, there is a continuing effort to increase device speed and increase device densities. Optical systems and technologies promise to deliver increasing speed and circuit packing density in the future. Optical waveguides typically include optical waveguide devices to provide optical functionality. Such optical waveguide devices can perform a variety of optical functions in integrated optical waveguide circuits such as optical signal transmission and attenuation.
In one aspect, optical waveguide devices include a variety of passive optical waveguide devices and/or a plurality of active optical waveguide devices. For example, certain gratings, lenses, filters, photonic crystals, and the like can be fabricated as passive optical waveguide devices. Similarly, active optical waveguide devices may function as filters, gratings, lenses, deflectors, switches, transmitters, receivers, and the like. Availability of a variety of passive and active optical waveguide devices and/or electronic devices provides a desired range of functionality. The availability of these devices is useful in making optical waveguide circuits simpler to design and fabricate.
A passive optical device does not change its function over a period of time excluding device degradations. A large variety of passive optical devices that include, e.g., optical fibers, slab optical waveguides, or thin film optical waveguides, may provide many optical functions. As such, the output or optical functionality of passive optical waveguide devices cannot be tuned or controlled. Additionally, passive active devices cannot be actuated (i.e., or turned on and off) depending on the present use of a region of an optical waveguide.
Many active optical waveguide devices such as modulators, filters, certain lenses, and certain gratings are precisely tunable. Tunability of certain active optical waveguide devices is important in making them more functional and competitive with present electronic circuits and devices.
Silicon-on-Insulator (SOI) and CMOS represent two technologies that have undergone a considerable amount of research and development relating to electronic devices and circuits. SOI technology can also integrate optical devices and circuits. It would be desirable to provide active optical waveguide device functionality and/or passive optical waveguide device functionality based largely on the CMOS devices and technology as well as manufacturing methods that allow for simultaneous fabrication of optically active and passive waveguide elements.
One embodiment of prior-art optical waveguide device is an arrayed waveguide grating (AWG) as shown in FIG. 2. The AWG 400 includes an input coupler 402, a plurality of arrayed waveguides 404, and an output coupler 406. The AWG 400 can be configured as a wavelength-division demultiplexer (if light signals travel from the left to the right in FIG. 2) or a wavelength-division multiplexer (if light signals travel from the right to the left in FIG. 2). In the AWG 400, each arrayed waveguide 404 has a different length between the input coupler 402 and the output coupler 406. The difference in length between each one of the different arrayed waveguides 404 corresponds to an optical phase shift of m2π, where m is an integer for the central design wavelength of the light that is applied to the AWG 400. Since each arrayed waveguide 404 has a different length, the light passing through the longer arrayed waveguides arrives at the output coupler 406 later than the light passing through the shorter arrayed waveguides.
AWGs 400, however, are difficult and expensive to produce. Each arrayed waveguide 404 is measured and formed separately. The operation of the AWG 400 requires that the different arrayed waveguides 404 differ in length by a distance equal to an m2π optical phase shift for the central design wavelength that the AWG is designed to multiplex/demultiplex. The cross-sectional area and the material of each arrayed waveguide 404 of the AWG 400 is constant to maintain the effective mode index (or the propagation constant β) of the different arrayed waveguides 404, and therefore provide a uniform velocity of light traveling through the different arrayed waveguides. As such, in present designs, each arrayed waveguide 404 of the AWG 400: a) has precisely calculated and measured lengths; b) has the same precisely produced and measured cross-sectional areas; c) has different lengths, such that the difference between the successive lengths, Δl is such that βΔl=m2π; and d) is smoothly-curved through a gradual radius of curvature to reduce bending losses of light flowing through the arrayed waveguide 404. Due to these requirements, the AWG 400 is challenging to design and fabricate since it is difficult to ensure the precise relative lengths of each one of the arrayed waveguides 404. Both the precision requirements and fabrication tolerances place extreme requirements on the manufacturing process. These waveguides traditionally use different indices of glass to make the core and the cladding. Silicon is used in the fabrication process but does not participate in the optical function. A 6″ Si wafer may be able to accommodate 5-50 AWGs 400 depending on the design requirements and the available index contrast between the core and the cladding, which is generally of the order of a few percent. The waveguides in AWGs are designed to be polarization independent so that both the polarizations of the input light are more or less treated equally. Considerable time and human effort is therefore necessary to produce precise AWGs 400.
It would therefore be desirable to fabricate passive optical waveguide devices (such as AWGs) using standard CMOS fabrication techniques which, when combined with active optical functions such as a modulator on the same substrate, could form the basis of a WDM system on a chip. It would also be desirable to fabricate such passive optical waveguide devices as AWGs and interferometers in a manner that the lengths and shapes of the arrayed waveguides are simple to accurately calculate, measure, and produce. Furthermore, it would be desired to apply active optical waveguide devices as tuning devices associated with optical circuits including passive optical waveguide devices, wherein much of the fabrication errors inherent in passive optical waveguide devices or device degradation over time can be dynamically tuned out by tuning the associated active optical waveguide devices.